Blow molded containers made of thermoplastic materials have been used to package a wide variety of consumer products, such as cosmetics, shampoo, laundry, and food. For such containers, having a smooth exterior surface is appealing to users as it generally enhances aesthetics, e.g., improved glossiness. Such a smooth exterior surface of a container is mainly a result of the impression of the inner surface of the mold used in shaping the molded container during a blow molding process. While a smooth mold inner surface is desirable for producing containers with improved aesthetics, this requirement poses challenges including compromising ventilation during the blow molding process.
In the blow molding process, ventilation is crucial for container quality. Ventilation allows air between the parison (or preform) of plastic and the mold to escape when the plastic expands in the mold cavity. Poor venting can cause air entrapment between the expanding plastic and the mold to prevent the plastic from fully contacting the mold. This leads to unsightly deformations in the formed container. Moreover, poor venting leads to a significant temperature build-up in the mold cavity, which can cause issues like sticking of the plastic onto the mold or burn marks of the plastic (i.e., the small dark brown or black discolorations on the molded container due to excess heat). These issues are particularly problematic for thermoplastic materials like polyethylene (PE) or polypropylene (PP) because such materials generally have a lower melting point and are more likely to stick onto a mold (as compared to materials like polyethylene terephthalate (PET)). In order to solve the above issues caused by poor venting, the prior art uses molds having a rough inner surface. Micropores on these rough molds (typically made by sandblasting) allow air to migrate through the micropores to the mold vents as the plastic inflates thereby reducing the air pressure (between the expanding plastic and mold) as well as mitigating the temperature build-up in the mold cavity. However, a rough mold leads to an undesired rough surface of the formed container.
Thus, there is a need to provide improved exterior surface smoothness (thereby improving aesthetics) to containers made from thermoplastic materials having a lower melting point. In particular, the present invention enables the use of a smooth mold to produce a container from thermoplastic materials having a lower melting point.
It is another advantage of the present invention to utilize the same mold to produce a variety of containers from different thermoplastic materials each having different melting points (which traditionally require different molds of varying degrees of smoothness).
It is another advantage of the present invention to provide a container made under relatively high processing temperature whilst avoiding the formation of burn marks.
It is yet another advantage of the present invention to provide a container that has reduced crystallinity and thus has improved surface smoothness.
It is even yet another advantage of the present invention to provide a container whilst avoiding the formation of flow line defects to the exterior surface of the container.
It is even yet another advantage of the present invention to provide a container that is easy to open, i.e., requiring a relatively low torque to open the container.